Age-dependent modulation of hippocampal excitability by KCNQ-channels

Abstract

Recently, mutations of KCNQ2 or KCNQ3, members of the KCNQ-related K⁺-channel (KCNQ-channel) family, were identified as cause of benign familial neonatal convulsions (BFNC). However, the exact pathogenic mechanisms of age-dependent development and spontaneous remission of BFNC remain to be elucidated. To clarify the age-dependent etiology of BFNC, we determined age-dependent functional switching of KCNQ-channels, GABAergic- and glutamatergic-transmission in rat hippocampus. The effects of inhibitors of KCNQ-channel, GABA- and glutamate-receptors on propagation of neuronal-excitability and neurotransmitter release were determined by 64-channel multielectrode-dish (MED64), whole-cell recording, in vitro release technique and in vivo microdialysis biosensor, using rat hippocampus from day of birth (P0) to postnatal-day 56 (P56). Inhibition of KCNQ-channels enhanced depolarization-induced glutamate and GABA releases during P0–P7, but not during P14–P28. Inhibition of KCNQ-channels magnified neuronal-excitability propagation from P0 to P14: maximal at P3, but this effect disappeared by P28. GABA_A-receptor inhibition surprisingly reduced neuronal-excitability propagation during P0–P3, but not at P7. AMPA/glutamate-receptors inhibition reduced propagation of neuronal-excitability throughout the study period. KCNQ-channels inhibition shortened spike-frequency adaptation, but this stimulation was more predominant during P<7 than P>14. During the first week of life, KCNQ-channels performed as a predominant inhibitory system, whereas after this period GABAergic-transmission switched from excitatory to inhibitory function. Contrary, glutamatergic-transmission has acquired as excitatory function from P0. These findings suggest that the pathogenic mechanisms of age-dependent development and spontaneous remission of BFNC are, at least partially, associated with the interaction between age-dependent reduction of inhibitory KCNQ-channel activity and age-dependent functional switching of the GABAergic-system from excitatory to inhibitory action in neonatal CNS.

Introduction

Epilepsy is a common neurological disorder afflicting 1–2% of the general population worldwide (Hauser, 1997). Hereditary factors are closely involved in the etiology of epilepsy (Hirose et al., 2000a) and genetic abnormalities have been identified in a few familial epilepsy syndromes including benign familial neonatal convulsions (BFNC) (Hirose et al., 2000a, Hirose et al., 2000b). BFNC is characterized by clusters of generalized seizures exclusively afflicting neonates, with spontaneous remission (ILAE, 1989, Aso and Watanabe, 1992, Bye, 1994, Plouin, 1997). Furthermore, it is well established that seizures of BFNC neonates often include partial seizures (Aso and Watanabe, 1992, Bye, 1994, Plouin, 1997).

Several mutations of KCNQ2 and KCNQ3, members of the KCNQ-related K⁺-channel (KCNQ-channel) family, have been recently identified to be associated with BFNC (Biervert et al., 1998, Charlier et al., 1998, Singh et al., 1998, Hirose et al., 2000b). Molecular biological experiments demonstrated that KCNQ2, KCNQ3 and KCNQ5 are widely co-expressed almost exclusively in the central nervous system (CNS) including the hippocampus (Biervert et al., 1998, Singh et al., 1998, Schroeder et al., 1998, Cooper et al., 2000, Smith et al., 2001). KCNQ2 and KCNQ5 are thought to fully function, when they are assembled as a heterotetramer with KCNQ3, because KCNQ2/KCNQ3 and KCNQ5/KCNQ3 heterometric channels generate 15- and 5-fold larger current than the corresponding homometric channels, respectively (Biervert et al., 1998, Schroeder et al., 1998, Schroeder et al., 2000, Tinel et al., 1998, Wang et al., 1998, Cooper et al., 2000, Schwake et al., 2000, Lerche et al., 2000). The major role of the increase in current of heterometric KCNQ-channel is thought to be an increase in surface expression of this channel, since co-expression of KCNQ2 and KCNQ3 led to a large increase in the surface expression of both KCNQ2 and KCNQ3 (Schwake et al., 2000). In addition, recent immunohistological studies indicated that KCNQ2/KCNQ3 heteromeric channel is located on proximal dendrites and soma in human hippocampal pyramidal neuron (Cooper et al., 2000). Taken together with these evidences, the pharmacological and electrophysiological profiles (voltage-dependence and kinetics) of these heterotetramer KCNQ-channels [KCNQ2/KCNQ3 (Biervert et al., 1998, Schroeder et al., 1998, Tinel et al., 1998, Wang et al., 1998, Cooper et al., 2000, Schwake et al., 2000, Smith et al., 2001) and KCNQ3/KCNQ5 (Lerche et al., 2000, Schroeder et al., 2000)] suggest that these channels contribute to the formation of native M-current, which is an important inhibitory regulator of sub-threshold neuronal excitability in CNS (Zhu et al., 2000).

Abnormalities of either KCNQ2 or KCNQ3 identified in BFNC are associated with loss of function of KCNQ-channels (Biervert et al., 1998, Charlier et al., 1998, Singh et al., 1998, Hirose et al., 2000b, Schroeder et al., 1998). Several studies have provided support for the “imbalance hypothesis”, that epileptic seizures are preceded by a relative imbalance between excitatory (i.e. glutamatergic system) and inhibitory (GABAergic system) neurotransmission (Hirose et al., 2000a). Such imbalance consequently precipitates and propagates abnormal neuronal hyperexcitability in the CNS, i.e., epilepsy. Recently, mutations in GABRG2 (encoding GABA_A receptor γ₂ subunit) or GABRA1 (encoding GABA_A receptor α₁ subunit) were identified as a cause of generalized epilepsy with febrile seizures (GEFS+) (Baulac et al., 2001), febrile seizures (FS) (Wallace et al., 2001), childhood absence epilepsy (CEA) (Wallace et al., 2001) and juvenile myoclonic epilepsy (JME) (Cossette et al., 2002). Thus, deficient function of mutant KCNQ-channels seems to cause convulsion in BFNC, consistent with the “imbalance hypothesis” (Hirose et al., 2000a). However, the pathogenic mechanisms of the age-dependent development and remission of BFNC during the neonatal period remain to be explained in relation to dysfunction of KCNQ-channels (Hirose et al., 2000a). In addition, although long term prognosis of BFNC is considered benign, individuals with BFNC have a higher risk for subsequent epilepsies (11%) compared with the general population (Plouin, 1997).

To investigate the possible relationship between deficient KCNQ-channels and age-dependent etiology of BFNC, including development, spontaneous remission and propensity for subsequent epilepsies in BFNC, the present study determined the age-dependent functional switching of KCNQ-channels, GABAergic and glutamatergic transmission in immature rat hippocampus.